The fresh air vent is how exhausted air is resupplied in your home. Without these vents and the fresh air they supply dangerous gases would build up in your home as well as the potential for back draft.
Fire needs air in order to burn, a fresh air vent is like providing fuel so the fire doesn't smother itself out with it's own smoke and ash. It also lets the smoke escape
For the burning you need oxygen. A fresh air vent is to help the burning process and to make the smoke go towards the funnel and from there out and not inside you room.
Well since it is a fresh air vent then I assume that it is the vent that controls the amount of air that is allowed to flow into the fire. The more air the hotter the fire and the faster the wood burns.
